Organic thin film transistors (TFT) made of organic semiconductor materials have been studied with enthusiasm due to the following advantages:
1) diversity of the materials used and high flexibility in processes of manufacture, product forms and the like;
2) ease of large area electronic applications;
3) simple layer structures, which allow simple manufacturing processes; and
4) manufacture with low-cost manufacturing equipment. In manufacturing of organic TFTs, printing process, a spin coat process and a dipping process are examples of techniques used to form organic semiconductor layers. Accordingly, the organic TFTs can be manufactured at an exceptionally low cost compared to conventional TFTs made of Si semiconductor materials.
When organic TFTs are integrated, a pattern of electrodes needs to be formed. Patent Document 1 discloses a manufacturing method of a laminated structure including the steps of forming a wettability variable layer containing a material whose critical surface tension changes by application of energy, forming a pattern having areas with different critical surface tensions (i.e. a low surface energy area whose critical surface tension is low and a high surface energy area whose critical surface tension is high) by applying energy to a part of the wettability variable layer; forming a conductive layer on the high surface energy area by applying a liquid containing a conductive material to the surface of the wettability variable layer on which the pattern has been formed; and forming a semiconductor layer on the wettability variable layer.
Also, when organic TFTs are formed, an organic semiconductor layer pattern needs to be formed. If organic TFTs are integrated without the organic semiconductor layer pattern formed, an organic semiconductor layer is formed not only within the channel regions but also outside the channel regions, whereby an OFF current occurs when the organic TFTs are in operation, which results in an increase in power consumption. Such an OFF current also causes crosstalk when pixels are displayed. Note that, in creating conventional TFTs including Si semiconductor materials, photolithographic etching is performed to form a Si semiconductor layer pattern.
The pattern formation of the organic semiconductor layer can be achieved by applying a photoresist, exposing and developing a desired pattern to form a resist pattern, and stripping off the resist by etching while using the resist pattern as an etching mask. However, in the case where a polymer material is used as an organic semiconductor material, if a pattern is formed by applying a photoresist on the polymer material, transistor characteristics may be degraded. As for the photoresist, a material is used which is formed by dissolving a novolac-type resin having naphthoquinone diazides as photosensitive groups with an organic solvent, such as a xylene solvent or a cellosolve solvent. It is often the case that a polymer material is soluble in an organic solvent and the like included in the photoresist. Also in the case of using crystalline molecules, such as pentacene molecules, as the organic semiconductor material, the transistor characteristics can be degraded to greater or lesser degrees. Furthermore, the transistor characteristics may be lowered by a remover liquid, such as ethylene glycol monobutyl ether or monoethanol amine, used to remove the resist, or may be degraded when rinsed with deionized water after the resist is removed. Thus, it can be seen that there are many problems associated with the conventional pattern formation of the organic semiconductor layer using photolithographic etching.
Patent Document 2 discloses a method for manufacturing an organic transistor by combining the following processes appropriately: a process of applying first electric charges to a predetermined position on an application surface while applying second electric charges opposite in polarity to the first charges to a coating material to thereby move the electrically-charged material to a predetermined position by Coulomb's force; a process of forming a depression at a predetermined position on the application surface and applying the coating material to the application surface to deposit the coating material in the depression; and a process of forming a pattern by evaporating a solvent included in the coating material after application, and then forming transistors by irradiating the pattern with a laser.
Patent Document 3 discloses a transistor manufacturing method including the steps of providing an electrically conductive layer upon a substrate; providing over the conductive layer a mask having at least one window; etching the conductive layer through the window to form an opening in the conductive layer and sectioning predetermined parts of the conductive layer to thereby form a source and a drain of a transistor; depositing a conductive material through the window to form in the opening a metal gate of the transistor; forming a metal oxide dielectric insulating layer on the gate; and introducing a semiconductor material into a space between the source and the drain, a space over the gate, and a space between the source or the drain, and the gate to thereby form a semiconductor body of the transistor. Note that the etching is carried out such that the conductive layer is undercut at the periphery of the window so that the opening is larger in size than the window in a direction parallel to the surface of the substrate. The deposition of the conductive material is carried out by metal evaporation so that the periphery of the gate closely matches that of the opening, whereby the periphery of the gate is spaced apart from the source and the drain.
However, these disclosed methods leave the problems associated with low throughput, an increase in the cost of manufacturing and the like due to the increased number of process steps.
Regarding the pattern formation method, inkjet printing and dispenser printing are known as examples of such. With these printing methods, a pattern can be drawn directly, whereby the material usage rate can be dramatically improved. Application of inkjet printing or dispenser printing to the pattern formation of the organic semiconductor layer is likely to allow the manufacturing process to be simplified, enable the yield to be improved, and achieve cost reduction. For example, a polymer material soluble in an organic solvent is used as the organic semiconductor material to prepare an organic semiconductor material liquid solution (organic semiconductor ink). Using this liquid solution, an organic semiconductor layer pattern can be formed by inkjet printing.
However, spreading of the organic semiconductor ink after application is a problem. In general, a metal material used for electrodes has a high surface free energy, whereby each droplet of the organic semiconductor ink deposited on the electrodes spreads out. Even if the organic semiconductor ink is applied with aim on the channel region of a transistor, deposited droplets spread since the electrodes occupy a large area. As a result, it is difficult to control the pattern formation of the organic semiconductor layer.
On the other hand, in the case of forming a gate electrode by inkjet printing using Ag ink, there is a limit to making the width of the gate electrode small given the printing precision. Accordingly, the gate electrode overlaps the source/drain electrode to a greater extent, whereby the parasitic capacitance between these electrodes increases. Furthermore, since the charge mobility is generally low in an organic TFT compared to a TFT including a Si semiconductor material, the organic TFT has a low cutoff frequency, thereby being unable to operate at high speed.    Japanese Laid-open Patent Application Publication No. 2005-310962    Japanese Laid-open Patent Application Publication No. 2004-297011    Published Japanese Translation No. 2003-536260 of the PCT International Publication